1. Field of the Invention
This invention relates to the bleaching of chemical pulp. More particularly, the invention relates to initial bleaching of sulphate pulp, bleaching sequences and filtrate cycles related thereto. The initial bleaching sequence according to the invention comprises treating the pulp in order to reduce the content of transition metals present in the pulp, a first chlorine dioxide treatment (D0) of the bleaching, addition of alkali in the D0 stage after addition of chlorine dioxide to adjust the pH value to be neutral or basic, followed by washing, and an alkaline oxygen and peroxide stage (EOP) as a stage carried out subsequent to the washing.
2. Description of the Related Art
The bleaching of sulphate pulp is divided into initial and final bleaching. During the initial bleaching, most of the lignin present in the pulp is removed. In the final bleaching, the residual lignin still present in the pulp is removed, and pulp darkening coloured groups, chromophores, are converted into the non-light-absorbing form.
The initial bleaching of sulphate pulp using chlorine dioxide conventionally consists of an acid delignification stage, generally a chlorine dioxide stage D0, and an alkaline extraction stage E, which is often reinforced with oxygen and peroxide (EOP) or with one of these (EO or EP). In addition to delignification, the acid stage releases metals present in the pulp. Furthermore, hexenuronic acid groups consuming bleaching chemicals may be removed by a hot acid treatment of the pulp.
The D0 stage and E stage of the initial bleaching are separated from each other by intermediate washing. In the washing, dissolved organic matter, spent chemicals and metals being free in ionic form in acid conditions, are removed from the pulp.
A dosage of active chlorine in the D0 stage is often relatively high, usually more than half of the chlorine dosage of the whole bleaching stage. The reactions of chlorine dioxide in the D0 stage are rapid, consuming thus a main part of the charged chemicals in a few seconds. However, the retention time in the D0 stage is usually about 30 minutes to ensure the reaction of all chemicals and to achieve a kappa number after the D0 EOP stages being as low as possible.
The reactions of chlorine dioxide degrade the structures of lignin. The filtrates resulting from the D0 stage contain a part of the reacted lignin and the main part of the spent chlorides. A substantial part of the lignin reacted during the D0 stage will be converted into a dissolving form only in a following alkaline stage so that the filtrate resulting from the EOP stage contains a remarkable amount of dissolved organic matter as well as chlorine bound to the lignin during the D0 stage.
The transition metals, such as Fe, Cu and Mn, degrade peroxide, and should thus substantially be removed from the pulp, or their content should be reduced prior to a stage using peroxide, i.e. prior to the EOP stage. Most of the metals can be removed in the washing stage, when the pH is sufficiently low, approximately pH 3. Hence, in the washing stage following a first acid stage of the bleaching, metals being precipitated in alkaline conditions are removed from the pulp. At a higher pH, an effective removal of the metals requires the use of a chelating agent. Said first acid stage of the bleaching may also be a separate acid treatment (A) of the pulp prior to a first acid oxidative stage (D0). If the acid stage is carried out at a higher temperature than normal, at about 90° C., also hexenuronic acids may simultaneously be degraded and thus removed. The degradation of the hexenuronic acids also releases other metals which can be removed by washing.
The effluents resulting from the bleaching form a significant part of the effluents from a the whole pulp mill. The washing filtrates are circulated within a bleaching plant from stage to stage, if possible. There has been efforts to reduce the amount of effluents also by using the resulting filtrates for other processes of the mill, i.a. for washing of brown stock. Thereby the filtrates and the dissolved wood material and chemicals present in the filtrate are fed to a recovery process of chemicals. The filtrates resulting from the D0 stage contain a large amount of chlorides which are detrimental to a recovery process.
Therefore, there has been efforts to recover filtrates resulting from the alkaline stage carried out subsequent to the D0 stage. However, also these filtrates contain chlorine compounds, because a substantial part of the lignin reacted during the D0 stage dissolves only during the alkaline treatment. Hence, in an effort to reduce the consumption of alkali, the filtrate resulting from the E stage has been used as a washing and dilution fluid in a D0 stage washer. However, bleaching chemicals are consumed by the great amount of dissolved matter present in the filtrate resulting from the EOP stage. Hence, the filtrates resulting from the alkaline and acid stages of the initial bleaching are usually removed to effluent treatment.
In existing facilities, the EOP stage of the initial bleaching is carried out as a separate step, whereby there is an intermediate washing step between the D0 and EOP stages. Alkalizing subsequent to a chlorine stage has been searched in a case wherein the next stage is a first acid D stage of the final bleaching, but the chemical consumption has been remarkably high compared to an alkali stage separated by intermediate washing /1/. Cook /2/, has suggested a combination of the D stage and the oxidative alkaline stage of the initial bleaching without an intermediate washing step. Ljungren /3/ has found that alkalizing combined to a chlorine dioxide stage reduces AOX discharges. In these three researches, the combination of the chlorine dioxide and the alkali stages is used to substitute an initial bleaching comprising separate chlorine or chlorine dioxide and alkali stages, whereby the next stage is a first acid D stage of the final bleaching, as is the case also in displacement bleaching.
In general, chlorine dioxide and peroxide are used for the final bleaching. In the final bleaching based on chlorine dioxide, the sequences D, DD and DnD are employed, either with or without intermediate washing steps between the stages. In the DnD sequence, the intermediate washing is carried out after the Dn treatment, but the neutralization after the D stage may be accomplished also without washing /4/. Suess et al. /5/ has researched the performance of the D and P stages of the final bleaching without intermediate washing between the stages. In a process of U.S. Pat. No. 3,884,752, neutralization carried out subsequent to the D1 stage is substituted for a previously generally used separate alkaline E2 stage. According to U.S. Pat. No. 4,238,281, the whole final bleaching is carried out without intermediate washing steps, DED.
In the displacement bleaching (pulse, dynamic bleaching) /6, 7, 8, 9/, the filtrate present in the pulp is displaced at the end of the stage by a filtrate resulting from next stage. The displacement bleaching is carried out using diffuser washers /10/. Therein the acid filtrate present in the pulp is displaced by the chemicals of next stage, and the reacted, but in acid conditions undissolved matter remains in the pulp, and the main part of dissolving matter remains in the pulp and is passed to next bleaching stage.